1. Field of the Invention
The invention relates to a non-intrinsically safely supplied measurement device with a connecting means having an “increased safety (E-xe)” type of protection for connecting the measurement device to a feed connection in order to supply the measurement device with electricity, an electronic means which contains the electronic components of the measurement device, a display means, a sensor made with “increased safety (Ex-e)” or “intrinsic safety (Ex-i)” type of protection and an output circuit that is not made with at least “intrinsic safety (Ex-i)” type of protection.
2. Description of Related Art
In the operation of an electrical device in an explosive atmosphere, the ignition danger is due to electrical sparks on the switches and electrical transitions of the measurement device. Otherwise ignition can occur on a hot surface of the measurement device. In order to be able to operate an electrical measurement device in an explosion-prone region, therefore explosion protection measures are necessary.
In Europe, explosion protection is regulated by the ATEX standard. Explosion protection for electrical devices is established in standards DIN EN 50014 to 50028 and EN 60079. The principles of explosion protection are described in standard DIN EN 50014, while the following standards describe individual types of protection. Finally, standard FIN EN 60079 describes installation of apparatus made with a certain type of protection at the site of their use.
Basically, it holds that wherever ignitable media, such as gases and vapors, occur in a high enough concentrations, and can mix with air or pure oxygen, there is the danger of explosion. These explosion-prone regions are typically found in and around plants of the chemical industry and mining facilities, but also in other regions, such as bottling plants for high-proof spirits or in grain silos.
However, basically, there are two different possibilities for counteracting explosions there, specifically so-called “primary” and “secondary” explosion protection. In this connection, “primary” explosion protection is based on the avoidance of flammable media. However, often flammable media cannot be avoided since the essence of a process or production method often lies in use of a certain flammable medium. In this case, then, measures of “secondary” explosion protection must be used, i.e., avoiding ignition sources which can ignite the explosive atmosphere.
The measures which are taken on electrical apparatus to prevent ignition of an explosive atmosphere surrounding them are divided into the above addressed types of protection. In this connection, one or more of these types of protection can be implemented to achieve the desired explosion protection.
In the initially described measurement device, a connection means made with an “increased safety (Ex-e)” type of protection for connection to a feed connection must be provided to supply the measurement device with electric power. Here, protection type “increased safety”, abbreviated “Ex-e”, indicates that measures are taken which reduce the probability of spark formation and elevated temperature. Thus, in normal operation, there cannot be any ignition source present. Typically, this is achieved by over-dimensioning of the parts used and by providing relatively large distances between locations between which a spark might pass.
Otherwise, in the initially described measurement device, there should be a sensor which is made, for example, in “intrinsic safety” type of protection, abbreviated “Ex-i”. In this connection, the “intrinsic safety” type of protection describes measures in which the energy in the circuit is limited to values which do not allow unduly high temperatures and/or ignition sparks or arcs which have the ignition energy required for an explosion. Here, it is essential that intrinsic safety does not relate to individual equipment, but to the entire intrinsically safe circuit. This intrinsically safe circuit includes not only the connecting lines, but at least one intrinsically safe apparatus and the pertinent apparatus.
An intrinsically safe apparatus is defined as an apparatus in which, by definition, all circuits are intrinsically safe. This means that the voltage and current in the intrinsically safe circuit are so small that, in a short circuit, interruption or ground fault, no ignition can take place, i.e., the ignition energy is smaller than the minimum ignition energy for igniting the explosive environment. In contrast to the pertinent apparatus described below, intrinsically safe apparatus are suited for operation directly in the explosion-endangered area.
The pertinent apparatus within the framework of “intrinsic safety” type of protection is defined as an apparatus in which not all circuits are intrinsically safe. However, the pertinent apparatus contains at least one intrinsically safe circuit which can be routed into the explosion-endangered region. Generally therefore, in the pertinent apparatus, a non-intrinsically safe signal is converted into an intrinsically safe signal. Thus, the pertinent apparatus can be, for example, an apparatus made with a different type of protection.
For non-intrinsically safely supplied measurement devices with an electronic means containing electronic components of the measurement device, as described initially, this electronic means is often made in “explosion-proof enclosure” type of protection, abbreviated “Ex-d”; this means that the electronic means is provided with an enclosure which, in the case of ignition within the enclosure, ensures that the housing of the enclosure withstands the pressure, and transmission of the explosion within the enclosure into the exterior is precluded by the gap widths of the openings out of the enclosure having to be kept correspondingly small, i.e., for example, to implement penetrations for electric leads.
The execution of the electronic means in “explosion-proof enclosure” type of protection has a host of disadvantages. Such an enclosure makes the electronic means very heavy, otherwise there also being very little freedom with respect to shape. Explosion-proof enclosures are generally made round or spherical for reasons of stability. Moreover, due to the indicated, only small allowable gap widths, complex cable penetrations for connections of other components to the electronic means must be provided.